The invention relates to a process for producing sorbents based on smectites for absorbing liquids. While these sorbents are suitable for absorbing any liquids; they are especially suited as litter for pets.
Keeping pets in a largely urbanized environment increasingly requires the use of litters. Litter is designed to absorb the liquids released by the animals and in doing so to diminish or prevent the development of odor as much as possible. Furthermore, it is designed to remove moisture from the semisolid, wet animal excrement and excretions to reduce or prevent odor development. Good litters moreover have the property of absorbing the released or removed liquids while forming compact lumps which can be easily and economically removed. Thus, an optimum animal litter has high absorbency.
Products currently found on the market include organic substances such as straw, sawdust, wood shavings, bark, porous plastic beads, shredded paper, cellulose fibers, agricultural waste, polyacrylates, etc. They are used alone or in combinations with inorganic materials. The disadvantage of the organic litters is often unsatisfactory lump formation or their consistency and their tendency to bacterial decomposition, especially in interaction with moisture.
On the other hand, DE-A-3 816 225 discloses litters which bind odor and moisture, especially for small animals, with a cellulose-containing material. The cellulose-containing material as a wet substance is mixed at least with a capillary-active, small-grain material with a grain size of less than 0.5 mm, in a weight ratio of 90:10 to 20:8. For example, the wet mass (recovered paper stock) which is formed from the treatment of waste waters of paper manufacture with a moisture content of 50 to 60% by weight can be used as the cellulose-containing material. As the capillary-active material, for example, bentonite can be used which in its untreated state has a moisture content of roughly 30% by weight. However pumice powder with a moisture content of at most 30% by weight is preferred. The resulting litter must be dried; which is very energy-intensive.
DE-A-37 28 812 and DE-C-38 25 905 disclose sorbents based on clays and clay minerals with a content of a mineral from the group of smectites which are sintered at temperatures of more than 650.degree. C. to achieve a high pore volume. Organic substances such as sawdust, cork, rice husks or the like can be added as porosity-enhancing agents. These porosity-enhancing agents however are burned out during heating and no longer remain in the product. The material obtained after burning no longer swells.
U.S. Pat. No. 5,188,064 discloses a process for improving the sorption capacity of swellable smectite clays with a particle size of at least 95%&lt;200 microns. This clay is mixed with a particulate cellulose-containing material with a particle size of at least 95%&lt;2000 microns. The weight ratio between the cellulose-containing material and smectite is roughly 0.05 to 3:1. Components can be mixed with one another either dry or wet. The mixture obtained after the drying process is extremely dusty. The cellulose component is not bound to the product so that the material tends to adhere strongly to animal fur and paws. This mixture cannot be used as litter. The mixture obtained after the wetting process has a higher density and lower sorption capacity compared to a wet kneaded mixture.
DE-C-3 034 169 discloses a water absorbing insert for food packages which contains, in a bag of a water-permeable film, a mixture of diatomaceous earth and an organic gel-forming agent such as carboxymethyl cellulose, cellulose ether, polyvinyl pyrolidone, starch, dextrose, gelatin and/or pectin. The mixture contains neither smectite nor calcium sulfate.
WO 86/00496 discloses a sorbent which is suitable as litter for pets and which is produced by mixing clay with cellulose-containing material such as rice husks or wheat bran, pelletizing the mixture, drying and heating the pellets to a temperature from 550.degree. to 1100.degree. C. At these temperatures the cellulose-containing material is burned. Moreover, the sorbent contains no gypsum.
DE-A4 028 933 discloses a process for producing a material for absorbing liquids, especially animal excrement, in which cellulose materials as fillers are first crushed coarsely and then finely until they have a wadding-like form. Thereafter they are mixed and pressed dry with a clay powder. The clay powder is obtained essentially from alumina minerals based on kaolinite and quartz. The material contains no gypsum.
DE-A4 101 243 discloses an animal litter and a process for its manufacture in which a cellulose material is mixed with weighting materials, thickeners and/or binders as well as water. The product is then dewatered, crushed and dried. The cellulose material can comprise, for example, wood, wood products, residues from wood processing and straw. Thickeners, for example, silicic acid, silicate materials, zeolite, bentonite and polyacrylates can also be used. No water content is indicated for the bentonite.
DE-A-3 644 826 discloses a material for absorbing liquids, especially an animal litter, which is characterized by a mixture of fillers, gypsum and accelerators. The fillers can consist of cellulose, especially waste paper and/or textile materials. As accelerators, gypsum or mixtures of gypsum and other inorganic sulfates can be used. The disadvantage of this process is that the cellulose-containing material must be dried beforehand and converted into a wadding-like form in an expensive process step.
An absorption material for liquids based on sewage sludge from the paper industry is produced according to EP-A-0 115 898 and EP 0 169 946. This sewage sludge, which contains cellulose fibers and inorganic pigments, is mixed with a surface-active component, dewatered, granulated, screened and dried to a water content from 2 to 5% by weight. The drying process is relatively expensive and undesirable due to the danger of fire. Moreover, in contact with moisture, the material does not form lumps.
U.S. Pat. No. 4,570,573 discloses a cat litter which contains roughly 60 to 94% by weight of an absorption agent, roughly 1 to 35% by weight of gypsum, and roughly 3 to 12% by weight water. Clay and paper can be used as the absorption agent. The gypsum is present in the form of a calcium sulfate dihydrate which is weakly acid and which reacts with urea to form a urea sulfate salt. Clay is generally defined as a material which contains kaolin as its main component.
The object of the invention is a process for producing sorbents based on smectites for absorbing liquids, in which as high a proportion as possible of wet recovered paper stock can be used without the product then needing to be dried to increase sorption capacity.